The present invention relates generally to WDM and DWDM communication systems, and more generally to a polarization interleaver employed in such systems.
Optical wavelength division multiplexing (WDM) and dense wavelength division multiplexing (DWDM) have gradually become the standard backbone networks for fiber optic communication systems. WDM and DWDM systems employ signals consisting of a number of different wavelength optical signals, known as carrier signals or channels, to transmit information on optical fibers. Each carrier signal is modulated by one or more information signals. As a result, a significant number of information signals may be transmitted over a single optical fiber using WDM and DWDM technology.
One approach to increasing fiber optic capacity is to use more closely spaced channels. For example, at one point in time, 200 GHz spacing was common for optical channels. As the state of the art improved, 100 GHz spacing were more commonly used for optical channels. Unfortunately, wavelength division multiplexed transmission systems are susceptible to performance limitations due to polarization dependent effects such as cross-talk between the multiplexed channels. Cross-talk, which is primarily caused by the non-linear index of refraction of optical transmission fibers, increases as the channel spacing decreases. Four-wave mixing is one significant deleterious effect that produces cross-talk.
One way to reduce four-wave mixing while simultaneously increasing spectral efficiency is to launch adjacent channels with orthogonal polarization states. An optical device that can be used to combine two sets of orthogonally polarized channels into one densely packed set with half the channel spacing is referred to as a polarization interleaver. Conventional polarization interleavers typically require a number of components such as multiple wavelength combiners or arrayed waveguide gratings, increasing their complexity and their cost.
Accordingly, it would be desirable to provide a polarization interleaver that requires a minimum number of optical components and which can be produced relatively inexpensively.
In accordance with the present invention, a method and apparatus provides a WDM optical signal having a plurality of channels with a pair-wise orthogonal polarization state. The method begins by receiving a plurality of unpolarized optical wavelengths defining a plurality of optical channels separated by a prescribed channel spacing. A polarization wavelength dependent shift is imparted to the optical wavelengths, which is substantially equal to a particular fraction of the prescribed channel spacing.
In accordance with one aspect of the invention, the particular fraction of the prescribed channel spacing is approximately equal to one half the prescribed channel spacing.
In accordance with another aspect of the invention, the prescribed channel spacing is uniform across the plurality of optical channels. Alternatively, the prescribed channel spacing may be non-uniform across the plurality of optical channels.
In accordance with yet another aspect of the invention, a polarization interleaver employing an arrayed waveguide grating is provided. The arrayed waveguide grating includes a plurality of input waveguides each for receiving a plurality of unpolarized optical wavelengths defining a plurality of optical channels separated by a prescribed channel spacing. A first free space region is optically coupled to the plurality of input waveguides and a set of arrayed waveguides are optically coupled to the first free space region. A second free space region is optically coupled to the set of arrayed waveguides and an output waveguide is optically coupled to the second free space region. The arrayed waveguide grating has a polarization wavelength dependent shift that is substantially equal to a particular fraction of the prescribed channel spacing. In accordance with another aspect of the invention, the particular fraction of the prescribed channel spacing is approximately equal to one half the prescribed channel spacing.